This invention relates to semiconductor devices, and particularly to those of the class employing nitrides or derivatives thereof as semiconductors, as typified by light-emitting diodes (LEDs) and transistors.
Nitride-based semiconductor devices have been known and used extensively which are built upon substrates of sapphire, silicon, or silicon carbide. Of these known substrate materials, silicon offers the advantages of greater ease of machining and less expensiveness than the others. Additionally, unlike sapphire, silicon permits doping to provide a substrate that is sufficiently electroconductive to serve as part of the main current path in the device. Offsetting these advantages of the silicon substrate, as incorporated in semiconductor devices of prior art design, was a comparatively great voltage drop across the potential barrier between the substrate and the nitride semiconductor region thereon. As a result, in the case of LEDs for example, an unnecessarily high drive voltage was required for obtaining light of desired intensity.
Japanese Unexamined Patent Publication No. 2002-208729 teaches an LED explicitly designed to resolve the noted difficulties accompanying the silicon substrate. Successively grown by epitaxy on an n-type silicon substrate according to this prior art are an aluminum nitride (AIN) buffer layer, an n-type indium gallium nitride (InGaN) layer, an n-type gallium nitride (GaN) layer, an InGaN active layer, and a p-type GaN layer. In the course of the epitaxial growth of these layers, there are diffused into the silicon substrate both indium and gallium from the InGaN layer and aluminum from the AIN buffer layer. The result is the creation of a Ga—In—Al—Si alloy layer to a certain depth from the surface of the silicon substrate. Lowering the potential barrier of the heterojunction between the silicon substrate and the AIN buffer layer, the Ga—In—Al—Si alloy layer serves to reduce the drive voltage required by the LED for a current of given magnitude. Less power loss and higher efficiency are thus accomplished.
Despite the creation of the alloy layer as above, the potential barrier between the n-type silicon substrate and the overlying nitride semiconductor layer has still been not nearly so low as can be desired. The drive voltage required by the silicon-substrate LED with the alloy layer has been approximately 1.2 times as high as that of the more conventional sapphire-substrate LED for given output light intensity. The same problem has affected not just LEDs but transistors and other semiconductor devices in which current flows through the silicon substrate in its thickness direction.
Another problem with the LED concerns an electrode configuration on its light-emitting surface. The electrode must be so constructed and arranged on the light-emitting surface as to meet the dual, somewhat contradictory requirements of how to produce light of utmost intensity from the light-emitting surface and how to expedite electric connection of the electrode to wire or like conductor. A well known electrode configuration is to overlay the light-emitting surface with a transparent electrode, as of a mixture (hereinafter referred to as ITO) of indium oxide (In2O3) and stannic oxide (SnO2) and to place an opaque wire-bonding pad centrally on the transparent electrode. The transparent electrode takes the form of a film of electroconductive material as thin as, say, 10 nanometers. The metal from which is made the bonding pad is easy to diffuse into the transparent electrode and, possibly, further into the underlying semiconductor region, with the consequent creation of a Schottky barrier between the bonding pad and the semiconductor region.
Capable of interrupting the forward current of the LED, the Schottky barrier reduces the amount of current flowing through that part of the semiconductor region which lies right under the bonding pad. The result is the flow of a correspondingly greater amount of current through the outer part of the semiconductor region which is out of register with the bonding pad. Thus the Schottky barrier functions just like the familiar current-blocking layer that has been used in LEDs for blocking current flow through the part of the active layer which immediately underlies the bonding pad. The current flowing under the bonding pad is a waste of energy, the light generated there being interrupted by the opaque bonding pad on the light-emitting surface of the LED. Just as the current-blocking layer contributes to enhancement of LED efficiency by causing a greater amount of current to flow through the outer part of the semiconductor region, so does the Schottky barrier.
The Schottky barrier has proved to possess its own drawback, however. As has been stated, the LED with an n-type silicon substrate has a comparatively high forward drive voltage. The higher forward drive voltage inevitably incurs greater power loss, and a greater amount of heat generated, at the silicon substrate and the semiconductor region. The Schottky barrier performs its intended function to a lesser extent when the device heats up to excessive temperatures than when it does not. As a consequence, in the prior art LED with the n-type silicon substrate, greater current leakage occurred through the Schottky barrier, resulting in turn in less current flow through the desired outer part of the semiconductor region.
The current-blocking layer is less temperature-dependent and so more reliable than the Schottky barrier for the purpose for which it is intended. However, the current-blocking layer is objectionable by reason of the additional manufacturing steps required for its own creation, in contrast to the Schottky barrier which demands no such steps, it being a byproduct, so to say, of the conventional transparent electrode and opaque bonding pad thereon.